My records indicate that Thomas Britt married Martha King Britt in Bertie County, NC in 1793. Also, that she was most likely born in 1772. There is a record of them being married in Bertie County.
It seems likely that the Martha Britt listed in 1850 in the Census is not the same Martha King Britt that Thomas married in 1793. If she was born in 1772, this would have made her 48 when she had her youngest son. Also, my dates for at least the three oldest children, is earlier than the NC census. Pashann, b. 1812, Blackmon, b. 1813, and Henry b. 1815. The 1880 Federal Census lists Blackmon as being 67 years old, which would make him having been born in 1813.
Do you know the maiden name of the Martha Britt listed?
I believe that most of the children were from a second marriage to someone also named Martha, a pretty common name.
How are you related to Thomas Britt?